[firebae_messaging] Support add-to-app

[lukaszciastko]

Currently, Firebase Messaging doesn't work or works partially (depending on the platform and type of a message: background or foreground) in an add-to-app scenario. If Flutter is embedded inside a native app and proceeded by a native screen, some notifications are not being delivered. Please, add support for this scenario (for example, by providing a simple to use relay mechanism).